1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, etc., which includes a plurality of developing means for developing an electrostatic latent image formed on an image bearing member by an electrophotographic method and an electrostatic recording method.
2. Related Background Art
FIG. 1 shows a schematic sectional view of a color image forming apparatus that has hitherto been utilized. In this example, an upper part of this color image forming apparatus is a digital color image reader portion 101 for reading image data from external information, and a lower part thereof is a digital color image printer portion 102 for forming an image on a recording material on the basis of the image data read by the digital color image reader portion 101.
In the image reader portion 101, an original 30 is placed on an original plate glass 31 and is exposure-scanned by an exposing lamp 32, and fluxes of image light reflected from the original 30 are converged at a full-color sensor 34 via a lens 33, thereby obtaining color separation image signals. These color separation image signals are transmitted through an (unillustrated) amplifier circuit, then subjected to processing in a video processing unit (not shown) and sent to the printer portion 102.
Installed in the printer portion 102 are the image bearing member and image forming means for forming the images on the image bearing member, such as a charging means, an exposure means serving as a latent image forming means, a developing means, a transferring means for transferring the image on the image bearing member onto the recording material defined as a transferring medium in this example, a fixing means for fixing the image to the recording material, etc., wherein a so-called image forming process is executed.
In this printer portion 102, a photosensitive drum 1 defined as the image bearing member is so held as to be rotatable in an arrowhead-direction. Disposed around the photosensitive drum 1 are a pre-exposing lamp 11, a corona charging device 2 defined as a changing means, a laser exposing optical system 3 as an exposing means, an electric potential sensor 12, four pieces of developing devices 4y, 4c, 4m, 4bk as developing means which respectively contain developers assuming different colors that are herein yellow, cyan, magenta and black, an on-drum light amount detecting means 13, a transferring device 5 as a transferring means, and a cleaning device 6.
To begin with, when forming the image by the printer portion, in an image forming process, the photosensitive drum 1 is rotated in the arrowhead-direction. The photosensitive drum 1, of which electric charges have been eliminated by the pre-exposing lamp 11, is uniformly charged by the charging device 2 in a charging process.
Next in a latent image forming process, in the laser exposing optical system 3, the image signals from the image reader portion 101 are converted into optical signals by a laser output portion (unillustrated), and thus-converted laser beams E are reflected by a polygon mirror 3a, then travel through a lens 3b and a mirror 3c and are projected on a peripheral surface of the photosensitive drum 1, whereby light images E assuming respective separation colors fall upon the photosensitive drum 1 with the result that the latent images are formed thereon.
In a developing process, the latent images on the photosensitive drum 1 are developed by operating predetermined developing devices, whereby developer images (toner images) of the developers, of which a base substance is resin are formed on the photosensitive drum 1. The developing devices are so constructed as to selectively, singly get close to the photosensitive drum 1, corresponding to each separation color as one of eccentric cams 24y, 24c, 24m, 24bk, which operate as moving members.
In a transferring process, the toner images on the photosensitive drum 1 are transferred onto a recording material P as a transferring medium supplied to a position facing the photosensitive drum 1 from a recording material cassette 7 via a conveying system 23 and the transferring device 5. The transferring device 5 in this example includes a transferring drum 5a, a transferring brush 5b, an attraction brush 5c for electrostatically attracting the recording material P, an attraction roller 5g facing the attraction brush 5c, an inside charging device 5d and an outside charging device 5e. A transferring sheet 5f, which is formed of a dielectric substance and bears the recording material P, is looped round in a cylindrical shape integrally in a way that pastes onto a connecting plate 5i in a peripheral surface opening area of the transferring drum 5a so axially supported as to be rotationally driven. The transferring sheet 5f involves using a dielectric sheet such as a polycarbonate film, etc. In this drum-shaped transferring device 5, as the transferring drum 5a is rotated, the toner images on the photosensitive drum 1 are transferred onto the recording material P defined as the transferring medium that is borne on the transferring sheet 5f by the transferring brush 5b serving as a transferring charging device. Thus, a desired number of color images (toner images) are transferred in superposition onto the recording material P carried while being attracted to the transferring sheet 5f, thereby forming a full-color image.
In the case of forming the full-color image, upon thus finishing the 4-color toner image, the recording material P is separated from the transferring drum 5a by dint of operations of a separation pawl 8a, a separation push-up roller 8b and a separation charging device 5h, and is discharged into a tray 10 through a heat roller fixing device 9.
On the other hand, subsequently to the transfer, residual toners on the surface of the photosensitive drum 1 are, after being cleaned by the cleaning device 6, reused for the image forming process.
By the way, if the apparatus remains unused for a long period of time, there decreases triboelectricity of the toners and of the developers in the developing devices. Therefore, for gaining stability by increasing the triboelectricity within the developing device as at a power-on time of the apparatus, a developing device preparation process for idle rotations (idling) of each developing device is provided in a preparation process before executing the aforementioned image forming process. Also, an execution means for executing this developing device preparation process is provided. The developing device preparation process is described in Japanese Patent Application Laid-Open Nos. H8-129277 and 2000-352869.
When the idling of the developing device is conducted, the developing device is made to face the photosensitive drum 1 charged at a predetermined electric potential in order to prevent a scatter of toners. Further, in the developing device preparation process wherein the developing device is subjected to the idling, after the idling has been finished, a density of a predetermined patch pattern developed on the photosensitive drum 1 is read. Gradations are corrected based on a result of the reading, and copy preparation operations from a power-on startup operation down to a possible-of-copying operation are conducted starting from the aforementioned developing device in accordance with a rotation.
FIG. 4 shows a flowchart of the copy preparation operations related to the developing device in the developing device preparation process at a power-on time.
After the power-on in step S1, the idling of the black developing device 4bk is started in step S2. When the idling of the black developing device 4bk is effected for a predetermined period of time, the idling of the yellow developing device 4y is next started in step S3. Similarly, the idling of the magenta developing device 4m and the idling of the cyan developing device 4c are sequentially performed in steps S4 and S5.
After the idling of the four-color developing devices has been finished, patch patterns exhibiting several types of predetermined densities are formed on the photosensitive drum 1 by the developing devices in steps S6 to S9. An on-drum light amount detecting means 13 reads the densities of these patch patterns formed thereon, and the gradation correcting operations are conducted in sequence for the respective colors. After finishing the developing device preparation process in steps S1 to S9, a normal multicolor image formation (full-color image formation) process is started.
In this type of full-color image forming apparatus, however, not only in the case of effecting the full-color image formation by way of a first mode but also in the case of performing the single-color image formation involving the use of only one predetermined developing device by way of a second mode, the four-color developing devices have hitherto been sequentially subjected to the idling in the developing device preparation process from the power-on operation down to the possible-of-copying operation in the same way as the developing device preparation process shown in FIG. 4. Therefore, even in the case of the single-color image formation, e.g., taking a black-and-white copy, the idling of the unnecessary developing devices was carried out, which required a considerable period of time.